Organic light emitting devices (OLED) generally can have two formats known as small molecule devices such as disclosed in commonly-assigned U.S. Pat. No. 4,476,292 and polymer OLED devices such as disclosed in U.S. Pat. No. 5,247,190. Either type of OLED device may include, in sequence, an anode, an organic EL element, and a cathode. The organic EL element disposed between the anode and the cathode commonly includes an organic hole-transporting layer (HTL), an emissive layer (EL) and an organic electron-transporting layer (ETL). Holes and electrons recombine and emit light in the EL layer. Tang et al. (Appl. Phys. Lett., 51, 913 (1987), Journal of Applied Physics, 65, 3610 (1989), and commonly assigned U.S. Pat. No. 4,769,292) demonstrated highly efficient OLEDs using such a layer structure. Since then, numerous OLEDs with alternative layer structures, including polymeric materials, have been disclosed and device performance has been improved.
Light is generated in an OLED device when electrons and holes that are injected from the cathode and anode, respectively, flow through the electron transport layer and the hole transport layer and recombine in the emissive layer. Many factors determine the efficiency of this light generating process. For example, the selection of anode and cathode materials can determine how efficiently the electrons and holes are injected into the device; the selection of ETL and HTL can determine how efficiently the electrons and holes are transported in the device, and the selection of EL can determine how efficiently the electrons and holes be recombined and result in the emission of light, etc. It has been found, however, that one of the key factors that limits the efficiency of OLED devices is the inefficiency in extracting the photons generated by the electron-hole recombination out of the OLED devices. Due to the high optical indices of the organic materials used, most of the photons generated by the recombination process are actually trapped in the devices due to total internal reflection. These trapped photons never leave the OLED devices and make no contribution to the light output from these devices.
A typical OLED device uses a glass substrate, a transparent conducting anode such as indium-tin-oxide (ITO), a stack of organic layers, and a reflective anode layer. Light generated from the device is emitted through the glass substrate. This is commonly referred to as the bottom-emitting device. Alternatively, a device can include a substrate, a reflective anode, a stack of organic layers, and a top transparent electrode layer. Light generated from the device is emitted through the top transparent electrode. This is commonly referred to as the top-emitting device. In these typical devices, the index of the ITO layer, the organic layers, and the glass is about 2.0, 1.7, and 1.5 respectively. It has been estimated that nearly 60% of the generated light is trapped by internal reflection in the ITO/organic EL element, 20% is trapped in the glass substrate, and only about 20% of the generated light can actually emit from the device and perform useful functions.
Madigan et al (Appl. Phys. Lett, Vol 76, No. 13, p 1650, 2000) taught the use of high index substrates with micro-lens to enhance the light extraction efficiency. Matterson et al (Adv. Mater. 2001, 13, No.2, p123-127), Lupton et al (Appl. Phys. Lett. Vol 77, No. 21, p3340, 2000) taught the use of corrugated substrates to improve light extraction. Garbuzov et al (Optics Letters, Vol. 22, No. 6, p. 396, 1997) taught the use of substrates with special shaped micro-structures to improve light extraction. Gifford et al (Appl. Phys. Lett. Vol. 80, No. 20, p. 3679, 2002) taught the use of substrates with periodical structure and opaque metal layer to enhance light coupling through surface plasmon cross coupling. All these methods, however, suffer the common problem of much increased complexity in the device construction and at the same time produce light outputs that have high angular and wavelength dependence which are not suitable for many practical applications.
Chou (International Publication Number WO 02/37580 A1) and Liu et al. (US 2001/0026124 A1) taught the use of a volume or surface scattering layer to improve light extraction. The scattering layer is applied next to the organic layers or on the outside surface of the glass substrate and has optical index that matches these layers. Light emitted from the OLED device at higher than critical angle that would have otherwise been trapped can penetrate into the scattering layer and be scattered out of the device. The efficiency of the OLED device is thereby improved but still have deficiencies.